One parameter commonly used to quantify the directionality of an electroacoustic device that receives sound (sound receiver), such as a hearing aid, is the directivity index D.
The directivity index quantifies the directivity by effectively comparing the reference-axis sensitivity of the device to the sensitivity of the device to sound arriving from all directions.
Directivity index measurements of hearing aids are made at NIST according to the general method specified in the relevant U.
When the directivity index measurement is implemented with the semi-aligned zone array sampling method that utilizes the forty eight sound source locations specified in the relevant U.
The directivity index determined in this manner for a hearing aid under simulated real-ear conditions is referred to as the simulated real-ear aided directivity index, and the directivity index determined in this manner for the open ear configuration of the manikin is referred to as the manikin unaided directivity index.
Uncertainties of the measurement results obtained for each one-third-octave frequency band of measurement with the NIST system are shown for the directivity factor in Table 3, and for the directivity index in Table 4.
Some focused on the importance of getting a good directivity index between the direct sound and an average of only vertical dispersion curves taken over wide angles around the speaker.
The curve is then used to generate a second directivity index again using the direct sound measurement as the other parameter.
To get the directivity index, the loudspeaker is enclosed a spherical shell with microphones placed at regular intervals.
Infinity keeps the directivity index smooth around the tweeter crossover point by employing a thin plastic waveguide placed over the tweeter.